1. Field of the Invention
This invention relates to a driving device for a moving body which effects forward and returning movements.
2. Description of the Prior Art
For example, in electrophotographic copying machines, an image original and a photosensitive medium are fixed in parallel relationship with each other and a movable optical system is reciprocally moved therebetween, whereby an optical image corresponding to the image original is projected upon the photosensitive medium to accomplish the exposure thereof.
Also, in copying machines wherein the optical system is stationary, an original carriage for supporting thereon an image original and a photosensitive medium are respectively moved to effect exposure.
The conventional means for effecting the forward movement of such a moving body at a predetermined speed and for effecting the returning movement thereof include those of the following types:
1. The type which controls both the forward and returning movements of the moving body by means of a clutch;
2. The type which controls only the forward movement by means of a clutch and controls the returning movement by a motive power; and
3. The type which controls only the forward movement by means of a clutch and controls the returning movement by the resilient force of a coil spring.
The control method mentioned under item (1) above is disadvantageous in that when reversed from its forward movement, the moving body suffers from a great shock since the speed of the returning movement is usually higher than that of the forward movement.
In the control method mentioned under item (2) above, the use of the motive power of a motor or the like to return the moving body results in a lower rising speed of the returning movement as indicated by a curve 1 in FIG. 1 of the accompanying drawings and this could not be said to be an effective method but was expensive.
The control method of item (3) above is inexpensive and simple in instrumentation, but the resilient force of the coil spring causes sharp acceleration of the moving body which continues until the moving body reaches its stopping position, as indicated by a curve 2 in FIG. 1, so that the moving body experiences a great shock at the stopping position. To alleviate such a shock, this method undesirable involves the necessity of providing a shock absorber which is complex and expensive. Especially, in an optical system of the type wherein the original carriage is movable, such a shock makes the position of the image original unstable.